linux-kernel - Re: [PATCH] ring_bufer: fix BUF_PAGE

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Date:	Thu, 18 Dec 2008 13:48:03 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ring_bufer: fix BUF_PAGE_SIZE


* Lai Jiangshan <laijs@...fujitsu.com> wrote:

> 
> impact: make BUF_PAGE_SIZE changeable.
> 
> Except allocating/freeing page and the code using PAGE_MASK,
> all code expect buffer_page's length is BUF_PAGE_SIZE.
> 
> This patch make this behavior more concordant.
> 
> Signed-off-by: Lai Jiangshan <laijs@...fujitsu.com>
> ---
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 668bbb5..0cf6caf 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -158,6 +158,10 @@ struct buffer_page {
>  	void *page;			/* Actual data page */
>  };
>  
> +#define BUF_PAGE_ORDER 0
> +#define BUF_PAGE_SIZE (PAGE_SIZE << BUF_PAGE_ORDER)
> +#define BUF_PAGE_MASK (~(BUF_PAGE_SIZE - 1))
> +
>  /*
>   * Also stolen from mm/slob.c. Thanks to Mathieu Desnoyers for pointing
>   * this issue out.
> @@ -165,7 +169,7 @@ struct buffer_page {
>  static inline void free_buffer_page(struct buffer_page *bpage)
>  {
>  	if (bpage->page)
> -		free_page((unsigned long)bpage->page);
> +		free_pages((unsigned long)bpage->page, BUF_PAGE_ORDER);

hm, why? Non-order-0 allocations are pretty evil - why would we ever want 
to do them?

	Ingo
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